This invention relates to a GPS (Global Positioning System) positioning method and a GPS reception apparatus to which the method is applied.
In a GPS system wherein a plurality of artificial satellites called GPS satellites are utilized to measure the position of a moving body, a spread spectrum modulation system is used as a modulation system for signal waves from the artificial satellites. For example, in a consumer GPS receiver, spread spectrum signal waves called C/A codes (Course Acquisition Codes) from GPS satellites (Navistar's) are received to perform positioning arithmetic operation.
The C/A code is formed from a code of a PN (Pseudo random Noise) series whose transmission signal rate is 1.023 MHz, for example, a Gold code. The code of the PN series repetitively appears with one period thereof provided by 1,023 chips (accordingly, one period=1 millisecond) as seen from FIG. 13A.
The code of the PN series of the C/A code is different among different satellites, but it can be detected by a GPS receiver in advance by which satellite a code of any given PN series is used. Further, such a navigation message as hereinafter described allows a GPS receiver to discriminate a signal from which satellite the GPS receiver can receive at the spot and at the point of time. Accordingly, if it is tried to perform, for example, three-dimensional positioning using a GPS receiver, then the GPS receiver receives radio waves from four or more satellites which can be caught at the spot and at the point of time and performs spectrum despreading of the radio waves and positioning arithmetic operation to determine the position of the GPS receiver itself.
As seen in FIG. 13B, one bit of satellite signal data is transmitted as codes of a PN series for 20 periods, that is, in a unit of 20 milliseconds. In other words, the data transmission rate is 50 bps. When the bit is “1”, 1,023 chips of codes of a PN series for one period have inverse values to those when the bit is “0”.
As seen from FIG. 13C, in the GPS system, one word is formed from 30 bits (600 milliseconds). Then, one subframe (6 seconds) is formed from 10 words as seen from FIG. 13D. As seen from FIG. 13E, a preamble which normally has a prescribed bit pattern even when data are updated is inserted in the top word of one subframe, and data are transmitted following the preamble.
Further, one frame (30 seconds) is formed from 5 subframes. A navigation message is transmitted in a data unit of one frame. The first 3 subframes of data of one frame include information unique to each satellite called ephemeris information. The information includes parameters to be used for determination of a trajectory of the satellite and a signaled time of the signal from the satellite.
In particular, the second words of the three subframes of the ephemeris information include information of a time from a week called TOW (Time of Week). Accordingly, the TOW of each subframe is information updated at intervals of 6 seconds.
All of the GPS satellites include an atomic clock and use common time information, and a signal is signaled from each satellite in a unit of one second of the atomic clock. Further, codes of a PN series of each satellite are produced in synchronism with the atomic clock.
The trajectory information of the ephemeris information is updated after each several hours, and the information remains fixed until it is updated. However, if the trajectory information of the ephemeris information is stored in a memory of a GPS receiver, then the same information can be used with a high degree of accuracy for several hours. It is to be noted that the signaled time of a signal from each satellite is updated after each one second.
The navigation message of the remaining two frames of data of one frame is information called almanac information which is transmitted commonly from all of the satellites. The almanac information must be collected for 25 frames in order to acquire all of the information and includes rough position information of each satellite and information representative of which satellites can be used. The almanac information is updated after each several months, but remains the same information until it is updated. However, if the almanac information is stored in a memory of a GPS receiver, then the same information can be used with a high degree of accuracy for several months.
In order to receive a GPS satellite signal, a code of a PN series which is prepared in a GPS receiver and is same as the PN series of the C/A code used by a GPS satellite to be received is used to establish phase synchronism with the C/A code of the signal from the GPS satellite to catch the satellite signal, and the satellite signal is spectrum despread. A code of a PN series is hereinafter referred to as PN code. As phase synchronism with the C/A code is established and despreading is performed, bits are detected, and consequently, a navigation message including time information and so forth can be acquired from the GPS satellite signal.
A satellite signal is caught by a phase synchronism search of the C/A code. In the phase synchronism search, a correlation between the PN code of the GPS receiver and the PN code of a reception signal from a GPS satellite and, when the correlation is higher than a correlation value determined in advance, it is discriminated that the PS codes are in synchronism with each other. If it is discriminated that the PS codes are not in synchronism with each other, then the phase of the PN code of the GPS receiver is successively shifted one by one chip while a correlation of the PN code of the GPS reception signal to the PN code of the GPS receiver is detected for each phase to detect the phase with which synchronism can be established.
In this instance, the PN code of the GPS satellite is driven with a clock of a very high precision frequency. Accordingly, if the clock for driving a generator of a PN code prepared in the GPS receiver has a degree of accuracy substantially equal to that of a clock of a satellite, then if the PN code of the GPS receiver is shifted through 1,023 chips, i.e. one period of repetition of the PN code, then phase synchronism is obtained with some phase and a spread spectrum wave from the satellite can be caught.
The clock for driving the generator of the PN code of the GPS receiver is usually obtained by dividing the frequency of a reference frequency oscillator prepared in the GPS receiver. A high precision quartz oscillator is used as the reference frequency oscillator. However, the oscillation frequency of the reference frequency oscillator of the GPS receiver is usually fluctuated by a temperature variation or a secular change. Therefore, there is the possibility that the chip frequency of the PN code may be displaced between the satellite signal and the signal of the GPS receiver. Therefore, the GPS receiver performs a frequency search so that the oscillation frequency of the built-in reference frequency oscillator may be adjusted to the frequency of the spread spectrum signal from the GPS satellite taking a variation of the oscillation frequency of the built-in reference frequency oscillator into consideration.
FIG. 14 illustrates such a frequency search as just mentioned. In particular, such a phase synchronism search as described above is performed when the frequency of the clock signal for driving the PN code generator of the GPS receiver is a certain frequency f1. Then, if a phase with which synchronism is detected is not found in the phase search for all of 1,023 chips for which the phase synchronism search is performed, then, for example, the dividing ratio of the signal from the reference frequency oscillator is varied to vary the frequency of the driving clock signal to another frequency f2. Then, a phase search for 1,023 chips is performed similarly. This is repeated by successively changing the frequency of the driving clock signal stepwise as seen in FIG. 14. The operation described is a frequency search.
A frequency of the driving clock signal which can be considered to allow synchronization is detected by the frequency search, and final phase synchronization of the PN code is performed with the clock frequency. Consequently, even if the oscillation frequency of the quartz frequency oscillator has some displacement, the satellite signal can be caught.
By the way, in order to perform positioning arithmetic operation on the GPS receiver, the distance between the satellite and the receiver must be determined. In particular, the GPS receiver measures a time interval, that is, a signal arrival time interval, until a signal forwarded from the satellite at a certain time arrives at the GPS receiver and multiplies the time interval by the velocity of light 3×108 m/s to calculate the distance.
In order to measure the signal arrival time interval, it is necessary to establish precise time synchronism with a signal from the satellite and measure two kinds of time intervals. One of the two time intervals is time information shorter than one period of a spread code obtained by establishing phase synchronism with the C/A code, that is, time information shorter than 1 milliseconds. The other time interval is time information longer than one period of a spread code, that is, time information longer than 1 millisecond.
The time information shorter than 1 millisecond is obtained as a timing at which phase synchronism of the C/A code is established to catch the GPS satellite signal. In particular, since the spread code (PN code) of the satellite is in synchronism with its atomic clock, if phase synchronism of the PN code is established on the GPS receiver, that is, if synchronism of the C/A code is established, then information shorter than 1 millisecond of the arrival time interval of a radio wave from the satellite is obtained.
However, only if synchronism of the C/A code is established, only time information shorter than 1 millisecond is obtained, but time information longer than 1 millisecond is not obtained. Therefore, time information longer than 1 millisecond is necessitated. Conventionally, such time information longer than 1 millisecond is obtained by acquiring a navigation message included in a signal from the GPS satellite. In particular, time information longer than 1 millisecond is obtained by establishing phase synchronism with the preamble pattern in the navigation message and referring to the TOW to confirm the phase synchronism timing.
As described above, in the conventional GPS receiver, in order to catch a satellite signal, a frequency search is required due to a temperature variation or a secular change of the reference frequency oscillator provided in the GPS receiver. Since comparatively much time is normally required for the frequency search, there is a problem that much time is required until positioning arithmetic calculation is performed finally to measure the position of the GPS receiver at present.
Where the conventional time synchronization method described above is used, there is a problem that information of the preamble and the TOW for acquiring time information longer than 1 millisecond is obtained in a unit of a subframe, that is, only once in 6 seconds. Besides, in order to prevent erroneous locking, it is preferable to confirm information of the preamble and so forth usually two or more times. Therefore, the time required until final time synchronism is established after synchronism between the signal from the satellite and the C/A code is established is more than 6 seconds even if the time information the GPS receiver has is valid.
The time of more than 6 seconds makes an obstacle if it is tried to shorten the time required until positioning arithmetic operation is started after power is made available. Further, where it is intended to incorporate the GPS positioning system in a portable apparatus, although power saving is demanded, such power saving cannot be achieved sufficiently because the prior art requires much time until positioning arithmetic operation is started as described above.